go/zodb: LastTid -> Sync + Head
Today LastTid is ambiguous: does it return locally cached last transaction ID, or it performs round-trip to server to get more uptodate view of what has been last committed? As the result of this ambiguity some drivers (e.g. FileStorage, ZEO) return cached view, while other drivers (upcoming NEO, living on t branch) was doing the full round-trip. There are also situations where whether or not LastTid performs syncing to server affects semantics: for example if there are two ERP5 nodes and one node commits something into ZODB and conveys the fact that something has been committed via another channel, the second ERP5 node should have a reliable way to observe committed data. But currently there is no such way: on next DB.Open, even with at=0 meaning "use latest transaction" and with opt.NoSync=false, DB.Open uses storage.LastTid() as a way to sync and oops - for some drivers it won't result in real synchronization and thus opened connection could potentially have older view with not yet latest data. To fix this ambiguity require drivers to provide Sync functionality only. That should return ID of last transaction committed to storage data as observed from some time _afterwards_ Sync call was made. In other words for particular client-server case, Sync cannot return cached view of storage and has to perform round-trip to the server. At IStorage level we provide two entry points: Sync, to perform syncing, and Head that returns database head (last transaction ID) as viewed by local cache without any synchronization. Please see changes to IStorageDriver and IStorage interfaces for details. The rest of the changes are: - teach zodb.storage (the IStorage implementer) to actually implement Sync/Head. - adapt zodb.DB to perform Sync for !opt.NoSync case. - teach FileStorage to do proper syncing instead of returning local cache. - teach ZEO to perform "lastTransaction" RPC call for syncing instead of returning local cache. - adapt the rest of the tree to interfaces change.
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